Merge pull request #13 from treydock/override_destroy

Override destroy
Release 1.3.0

Signed-off-by: Ken Barber <ken@bob.sh>
- Allow overriding prefabs including using boxes on local system
- Allow defining a project specific .prefabs.yml
- Update README to document use of custom prefabs
- Relates to Issue #7
- Add back in text that still applies to README